In the past, various surface treatments have been carried out on aluminum heat exchangers and their fins, to provide a coating which would prevent the formation of "white rust", i.e., white corrosion deposits. These processes have included anodizing, hot water or steam treatment to form boehmite films, resin film treatments and the like. While the protective coatings produced by these processes have been effective in preventing or at least minimizing the formation of "white rust", the surfaces of these coatings have been substantially unwettable and, in many instances, have been water-repellent. Chromate films have also been used to provide corrosion protection. Although these films, initially, are, at least to some extent, water wettable, in the course of time, particularly under hot, dry conditions, the surface of these films change rom hhydrophilic to hydrophobic.
As is well known, although heat exchangers are designed to have the surface areas of the heating and cooling parts as large as possible in order to increase the heat radiation or cooling effect, they typically have very small or narrow spacings between the fins. As a result, particularly in the case of cooling, atmospheric moisture collects on the heat exchange surfaces, and particularly in the fin spacings. To the extent that the fin surface is hydrophobic, the collected water forms in drops, thus blocking the fin spacings and, thereby, increasing the air flow resistance and reducing the heat exchange efficiency. Additionally, the water drops accumulated in the fins spacings may be easily scattered by the blower of the heat exchanger and are, thus, apt to overflow from the water drip trays set up in the lower part of the heat exchangers and contaminate the area of the heat exchanger with water.
In order to prevent the blockage of the spacings between the fins by the residual water drops, the aluminum surfaces have heretofore been treated to make them hydrophilic and to increase their wettability. Generally, however, the treatments to increase wettability have not imparted corrosion resistance to the surfaces as well. Although the water would generally flow away from a surface which has been made hydrophilic without causing appreciable corrosion, the hydrophilic nature of the surfaces treated in accordance with the prior art have become easily impaired during the use of the heat exchanger. When this occurs, significant corrosion of the heat exchange surfaces results.
It is, therefore, an object of the present invention to provide a process for the treatment of aluminum heat exchanger surfaces which eliminates the problems which have heretofore been encountered in the art.
A further object of the present invention is to provide a surface treatment for aluminum heat exchangers which increases the wettability of the surfaces, while providing corrosion resistance and preventing the formation of "white rust".
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.